Closed center gerotor motors have many advantages, a number of which are described in Hollis White's seminal U.S. Pat. No. 4,877,383 (Device Having A Sealed Control Opening And An Orbiting Valve). This present invention is directed to an improvement to the closed center type devices wherein the seal for the central drive opening of the commutator valve is provided through the use of a moveable piston. This seals the opening while also providing additional advantages.